First, you'll need to make sure that the new batteries have the same specifications (voltage, capacity, discharge rate, etc.) as the old ones. If they don't, the battery pack may not function properly or could even be dangerous.
Second, the battery management system (BMS) in the pack may not be designed to work with the new batteries. The BMS is responsible for monitoring and controlling the charging and discharging of the batteries, and if it's not compatible with the new cells, it could cause problems.
Finally, opening up a battery pack can be dangerous if you don't know what you're doing. Lithium-ion batteries are volatile and can catch fire or explode if mishandled. If you're not experienced with working on electronics, it may be best to leave this task to a professional.
In summary, while it's technically possible to replace the batteries in a triangle battery pack, it's important to ensure that the new batteries are compatible with the pack's specifications and BMS. Additionally, caution should be taken when working with lithium-ion batteries, and inexperienced individuals should seek professional assistance.